In telecommunications systems, some equipment may be located in a remote location where accessing a power source to power the equipment is neither economical nor desirable due to cost or marketing considerations. In these situations, the remote equipment may be powered from a power source through the same type of subscriber loop twisted pair wires (subscriber loops) that are normally used to deliver telecommunication services from service provider equipment to subscriber premises. In this situation, the remote equipment is said to be “loop powered”. Remote equipment that may be loop powered may include remote terminals (RT), pairgain devices, loop extenders, network interface devices (NID), optical network termination (ONT) equipment, integrated access devices (IAD), and subscriber communication equipment such as a POTS (plain old telephone service) telephone, IP (Internet Protocol) telephone, FAX, set top box, or data modem.
FIG. 1 is a circuit diagram illustrating a typical environment 100 for loop powered remote equipment 110. The power source 102 that sources charge to the subscriber loop 104 is typically an earth referenced power supply with an output impedance of typically less than 5 ohms. The subscriber loop 104 is located physically in the external environment and may be subject to faults 106 from lightning 150 and mains power lines 160. Primary protectors 108 are provided on each wire at each end 103, 105 of the subscriber loop 104. The remote equipment 110 typically consists of a protection circuit 120 which includes protection electronics 170 and also a charge storage circuit 180 with an input impedance of typically less than 5 ohms. Subsequent electronics 130, 140 in the remote equipment 110 may include a power supply 130 that typically uses a transformer 190 to isolate the application electronics 140 from the high voltages that may occur on the subscriber loop 104 due to faults 106 and a regulator 191 to develop a stable power supply to power the application electronics 140.
Due to their physical location in the external environment, faults 106 may occur between the subscriber loops 104 and foreign potentials. These faults may include lightning 150 induced current and voltage, as well as power cross and induced AC current from mains power wires 160. For subscriber loops that are used for the delivery of mainstream telecommunications services to subscribers who use, for example, POTS telephones, modems, fax machines, and data modems, a number of systems are known for the protection of subscriber loop electronics that source charge on one end 103 of the subscriber loop 104, typically in the line card of the service provider, and electronics that sink charge from the other side 105 of the subscriber loop 104, typically the subscriber location. However, these systems are not applicable to subscriber loops that are used for the loop powering of remote equipment, in that the series impedance of electronics on each end of the subscriber loop utilized for loop powering, typically a few ohms, is considerably lower than the series impedance of the electronics on a subscriber loop used for mainstream applications, typically 100 ohms or more. For example, the currents developed from a lightning strike of 1000 V would be 20 times higher in the loop powered circuits, since the input impedance is 20 times smaller.
In general, primary protectors 108 are provided on subscriber loops for shunting charge to earth typically when the potentials across the primary protector exceed several hundred volts. All circuits connected to the primary protectors 108 must operate with consideration of the independent behaviour of these primary protectors 108.
The power supply 102 that sources charge to one end 103 of a subscriber loop 104 is typically earth referenced, thus protection circuits from faults beyond the primary protectors 108 may be designed using relatively simple circuits, or very often no additional circuits at all, that would shunt the fault energy locally to earth. Such protection circuits may be designed to maintain the connection between the power source 102 and the subscriber loop 104 throughout fault events, however this is not always the case.
Equipment 110 located remotely that is loop powered must sink current from a subscriber loop 104. However, such equipment cannot be earth referenced and typically prevents the conduction of current to earth for voltages within the primary protector activation voltage range. Furthermore, the energy that enters the remote equipment 110 may be common mode which occurs when a fault influences both wires of the subscriber loop twisted pair 104, or differential mode which occurs when one of the primary protectors activates prior the other primary protector on the subscriber end 105 of the subscriber loop 104. Thus, protection circuits for electronics that sink energy from a subscriber loop 104 and that form the power supply of remotely powered electronics equipment 110 are challenging to design effectively.
For the protection of remote equipment 110, beyond the primary protectors 108, existing systems typically use simple electronic circuits 170 that have several drawbacks. For example, fuses may be used, however, these require replacement by a service technician after every fault.
Thyristors may be used to activate prior to the primary protectors 108 or in coordination with the primary protectors 108, however as this design method requires the thyristors to conduct most of the energy in the fault event, the thyristors must therefore be quite large. Also, the voltage developed across the thyristors may have large peaks during the fault event that is presented across the subsequent electronics 130, 140, thus the subsequent electronics 130, 140 must be over-designed and expensive. Furthermore, standards bodies are now requiring more stringent testing, thus solutions based on thyristors have greater difficulty achieving compliance.
A relay or solid state switch to isolate the remote electronics 110 may be used that is activated when sensors detect a fault event. However, relays being mechanical are prone to wear and tear. In addition, if a relay or solid state switch is used to disconnect the subsequent electronics 130, 140 when a fault occurs and later reconnect to the subscriber loop 104 when the fault is cleared, a service interruption results. These protection circuits are thus expensive to deliver and maintain and result in interruption of service when faults occur.
Even new generation remote telecommunications equipment that is loop powered with copper subscriber loops, and that relies on optical fiber for all transmission, is subject to disruption of service if the remote equipment 110 is susceptible to outages due to fault conditions that affect the copper subscriber loop (i.e., but not the optical fiber). Such remote equipment 110 may require the addition of battery power if achieving minimum service disruption is an objective. This results in increased costs for capital equipment and maintainance.
A need therefore exists for an effective power and protection system for loop powered remote equipment. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages.